1. Field of the Invention
The present invention relates to an improved vehicle antenna system which can efficiently detect broadcast waves received by the vehicle body and transmit them to various onboard receivers.
2. Description of the Prior Art
With modern automobiles, antenna systems are essential for positively receiving broadcast and/or communication waves at various onboard receivers such as radios, TV's, car-telephones and others. Antenna systems also have an important role in citizen band tranceivers as providing communication between an automobile and the other stationary or movable station. In the future, such a vehicle antenna system will increasingly be important for vehicles standardized with various receivers.
A pole type antenna is known as one of the conventional vehicle antenna systems. The pole antenna projects exteriorly from the vehicle body and exhibits a favorable performance of reception in its own way. However, the pole antenna was always an obstruction in the design of the vehicle body.
The pole antenna also is disadvantageous in that it may accidentally or intentionally be subjected to damage and in that the pole antenna may produce unpleasant noises while the vehicle runs at high speeds. Therefore, it was very desirable to eliminate the pole antenna from the vehicle body.
Recently, the number of frequency bands for broadcast or communication waves to be received on the automobiles are being increased. If a plurality of pole antennas are located on a vehicle body matching the increased number of frequency bands, they would degrade the aesthetic concept of the vehicle appearance. Furthermore, there would be created an electrical interference between the pole antennas to also remarkably degrade the reception performance
Some attempts have been made to eliminate or conceal pole antennas. One such attempt involves applying an antenna wire to the rear window glass of the vehicle body.
Another attempt involves the use of a high frequency pickup which includes a loop antenna for detecting surface currents induced on the vehicle body by broadcast waves.
A conventional example of vehicle antenna systems utilizing such a loop antenna will now be described with reference to the drawings.
Referring to FIG. 6, there is shown an electromagnetic coupling type high frequency pickup 10 which includes a loop antenna 12 electrically connected with a variable capacity diode 14 and a pre-amplifier. FIG. 6 also shows a circuit including the variable capacity diode 14 and a receiver connected with the diode.
As can be seen from FIG. 6, the loop antenna 12 is connected in series with a capacitor C.sub.1, the variable capacity diode 14 and a capacitor C.sub.2. The total series capacity of these connected components determines a resonance frequency in the loop antenna 12. The output of the high frequency pickup 10 is taken out at one end of the capacitor C.sub.1 and at the anode end of the variable capacity diode 14 and then subjected to desired impedance conversion and high frequency amplification by the aforementioned pre-amplifier which is located near the pickup 10. As shown, the pre-amplifier includes a band pass filter (BPF) for eliminating undesirable signals such as noise signals and others to select signals belonging to a desired frequency band. High frequency signals detected by the band amplification are then subjected to an impedance conversion in an impedance converting circuit which consists of resistors and capacitors and further to a high frequency amplification. Thereafter, the signals are supplied to the receiver through a coaxial cable 18. The pre-amplifier receives a power voltage used to control the circuit through a cable 20.
Signals detected by the pre-amplifier are maximum at the resonance frequency of the high frequency pickup 10. The capacity of the variable capacity diode 14 is varied to bring the resonance frequency in line with a desired reception frequency. This permits a miniaturized antenna to receive broadcast waves very sensitively. In the illustrated conventional example, the pre-amplifier further includes a neon tube NL for protecting the semiconductor elements from high voltages due to thunderbolt or static electricity.
In order to vary the capacity of the variable capacity diode 14, a predetermined control voltage is applied to the cathode side of the variable capacity diode 14. Such a control voltage is controlled in connection with a tuned frequency in the receiver.
FIG. 6 further shows a portion of the receiver 22 which comprises an antenna terminal 24 connected with the other end of the coaxial cable 18. The antenna terminal 24 also is connected with the next reception circuit through a tuning circuit 26 via a capacitor 28. The tuning circuit 26 is adapted to vary the inductance of a coil or the capacity of a capacitor to select a tuned frequency. The tuned frequency thus selected is controlled and selected by a tuned frequency control circuit 30 and at the same time digitally displayed on a display 32 in the interior of the vehicle body. On the other hand, a tuned frequency control voltage is supplied to the cathode of the variable capacity diode 14 from the tuned frequency control circuit 30 of the receiver 22 through a variable resistor 34 and a resistor 36. Thus, the variable capacity diode 14 will be supplied with a control voltage corresponding to the tuned frequency selected by the tuning circuit 26.
When a desired reception frequency is selected at the receiver 22, the high frequency pickup 10 will be controlled to bring its resonance frequency in line with said tuned frequency for receiving broadcast waves belonging to the desired frequency band.
As described hereinbefore, the resonance frequency in the loop antenna of the high frequency pickup depends on the inductance of the loop antenna and the total capacity of series-parallel capacitors. The inductance of the loop antenna depends on its own opening area. The prior art vehicle antenna system is thus adapted to use a variable capacity diode to vary the capacity of the capacitor means such that the reception can be carried out through an increased range of bands. Since the resonance frequency of the loop antenna may vary, for example, due to variation of the power voltage in the vehicle, the prior art vehicle antenna system requires another power supply for stabilizing the resonance frequency in the loop antenna. This increases the size of the vehicle antenna system.